1. Field of the Invention
The present invention relates to antennas and dielectric substrate materials therefor, and in particular, to a tunable microstrip antenna dielectric material that is capable of use in portable or mobile applications where minimal aperture size and weight are desired, and where high bandwidth is preferred.
2. Description of the Related Art
U.S. Pat. No. 6,075,485 to Lilly et al. entitled xe2x80x9cReduced Weight Artificial Dielectric Antennas and Method for Providing the Samexe2x80x9d dramatically advanced the state of the art.
An artificial dielectric structure 10 according to U.S. Pat. No. 6,075,485 is shown in FIG. 1. It comprises a periodic structure or stack of alternating layers of high and low permittivity isotropic dielectric materials 12 and 14, having respective relative permittivities of ∈r1 and ∈r2. As shown in the drawing, layers 12 and 14 have respective thicknesses of t1 and t2, and the direction normal to the surface of the layers (i.e. the direction of stacking of the layers) is parallel with the Y axis. The number of alternating layers 12 and 14 used in the stack depends on their respective thicknesses and the overall size of the structure desired.
One of the merits of the structure of FIG. 1 is that tensor permittivities in the dielectric structure can be engineered to be any value between ∈r1 and ∈r2 by appropriate selection of the respective thicknesses for given respective permittivities of layers 12 and 14. The weight of the resulting structure 10 can be easily designed as well. In particular, a significant weight savings can be achieved by selecting a thin high permittivity dielectric material for layer 12 and a much thicker but very low weight dielectric material such as foam for layer 14.
Even greater weight savings can be achieved when the high permittivity dielectric material layer 12 is itself an artificial dielectric material, such as a frequency selective surface (FSS). For example, a 0.020xe2x80x3 thick FSS can be designed to represent an equivalent capacitance of up to ∈r=800, while exhibiting a specific gravity of only about xcx9c2.5 grams/cm3, further improving the results obtained in the above example.
As shown in FIGS. 2 and 3, a frequency selective surface (FSS) 20 for possible use as a high permittivity dielectric material 12 in structure 10 is an electrically thin layer of engineered material (typically planar in shape) which is typically comprised of periodic metallic patches or traces 22 laminated within a dielectric material 24 for environmental protection.
The electromagnetic interaction of an FSS with plane waves may be understood using circuit analog models in which lumped circuit elements are placed in series or parallel arrangements on an infinite transmission line which models the plane wave propagation. FSS structures are said to be capacitive when their circuit analog is a single shunt capacitance. This shunt capacitance, C (or equivalent sheet capacitance), is measured in units of Farads per square area. Equivalently, the reactance presented by the capacitive FSS can be expressed in units of ohms per square area. This shunt capacitance is a valid model at low frequencies where (xcex21t1)  less than  less than 1, and t1 is the FSS thickness. As a shunt capacitance, electromagnetic energy is stored by the electric fields between metal patches. Physical implementations of capacitive FSS structures usually contain periodic lattices of isolated metallic xe2x80x9cislandsxe2x80x9d such as traces 22 upon which bound charges become separated with the application of an applied or incident electric field (an incident plane wave). The periods of this lattice are much less than a free space wavelength at frequencies where the capacitive model is valid. The equivalent relative dielectric constant of a capacitive FSS is given as ∈r=C/(∈0t1) where ∈0 is the permittivity of free space. FSS structures can be made with ∈r values extending up to several hundred.
FIG. 2 is a top view of a conventional anisotropic FSS 20 comprised of square metal patches 22 where each patch is identical in size, and buried inside a dielectric layer 24 (such as FR-4). FIG. 3 is a cross-sectional side view of FIG. 2 taken along sectional line 3xe2x80x943 of FIG. 2. As shown, the gaps between patches 22 are denoted as gx in the x direction and gz in the z direction. If these variables are different dimensions, as shown in this figure, then the equivalent capacitance provided by the FSS is different for electric fields polarized in the x and z directions. Since gx is smaller than gz, the equivalent sheet capacitance for x-polarized E fields will be larger than for z-polarized E fields. For a given value of incident E field, more energy will be stored for the x polarized waves than for the z polarized waves. This leads to ∈rx greater than ∈rz in the FSS, and ∈x greater than ∈z in the equivalent bulk permittivity for a layered substrate when it is included in a non-homogeneous stacked dielectric substrate such as substrate 10 (assuming that the second layer is isotropic, such as foam).
FIGS. 4 through 6 illustrate a linearly-polarized patch antenna 40 according to U.S. Pat. No. 6,075,485. FIG. 4 is a top view, and FIGS. 5 and 6 are cross-sectional views taken along lines 5xe2x80x945 and 6xe2x80x946, respectively. As shown, antenna 40 includes a substrate comprised of artificial dielectric material 10, having alternating layers 12 and 14 of high and low permittivity dielectric materials, respectively, a microstrip patch 42, a coaxial feed 44 and a metalized ground plane 46.
To achieve the same resonant frequency in patch antenna 40, having an artificial dielectric material substrate, as in a conventional patch antenna with a homogeneous substrate, the artificial dielectric substrate is oriented so that the uniaxial axis, that is, the axis of anisotropy (where ∈x=∈z greater than  greater than ∈y, for example) is perpendicular to the surfaces of the high dielectric layers (the y axis in FIGS. 4 and 5, i.e. the direction in which the layers are stacked), and is parallel to the surface of the microstrip patch 42.
Antenna 40 can be, for example, a low weight UHF (240-320 MHz) patch antenna. For purposes of comparison, a conventional patch antenna for this application would include, for example, a homogeneous ceramic slab (8xe2x80x3xc3x978xe2x80x3xc3x971.6xe2x80x3) of material PD-13 from Pacific Ceramics of Sunnyvale, Calif. where ∈r=13 and the specific gravity is 3.45 grams/cm3. The weight of the homogeneous substrate having the required dimensions would thus be about 12.75 lbs. In the lightweight substrate design of U.S. Pat. No. 6,075,485, layer 12 of substrate 10 can be, for example, a 0.020xe2x80x3 thick FSS (such as part no. CD-800 of Atlantic Aerospace Electronics Corp., Greenbelt, Md. for example) designed to represent an equivalent capacitance of at least 300 for the x and z directions of FIG. 1. This FSS is made from one 0.020xe2x80x3 thick layer of FR4 fiberglass whose specific gravity is approximately 2.5 grams/cm3. To achieve an effective relative permittivity of ∈x=∈z=13∈0, layer 14 can be, for example, a 0.500xe2x80x3 thick Rohacell foam of the same type used in the example above. Substrate 10 having these design parameters weighs approximately 6.5 oz., which represents a 97% weight reduction from the conventional homogeneous substrate for this antenna application.
For fixed-frequency UHF applications as described above, patch 42 of FIG. 4 can be a six inch square patch (L=6xe2x80x3) printed on a 8xe2x80x3xc3x978xe2x80x3xc3x970.060xe2x80x3 thick Rogers R04003 printed circuit board (not shown). The circuit board is mounted face down so that patch 42 touches the ceramic slabs of the artificial dielectric substrate 10. The fixed frequency patch antenna 40 built according to these specifications resonates near 274 MHz with a clean single mode resonance. Radiation efficiency, as measured with a Wheeler Cap, is 82.2% (xe2x88x920.853 dB). Swept gain at boresight, and E-plane and H-plane gain patterns, also compare very similarly to the same patch with a homogeneous substrate. However, as shown above, the fixed frequency patch antenna having artificial dielectric substrate 10 weighs about 97% less than the patch antenna having a conventional homogeneous substrate.
U.S. Pat. No. 6,075,485 achieved remarkable weight and size reductions for a higher frequency antenna, which is desirable for many applications such as autos, aircraft and spacecraft. However, even further benefits may be desired that are not provided solely thereby.
For example, U.S. Pat. No. 6,075,485 taught that a tunable patch antenna such as that described in U.S. Pat. No. 5,777,581 could be used with the artificial dielectric substrate to provide a small, lightweight antenna capable of tuning over the military fleet SATCOM band: 240 MHz to 320 MHz, a tuning ratio of 1.33:1. Such antennas use PIN diodes to expand or contract the effective electrical size of a cavity-backed patch antenna. However, further development work has not been able to extend the tuning ratio beyond about 1.5:1. It would be desirable to find a way to electronically tune a conformal UHF antenna over at least a 2:1 bandwidth, so as to be usable for the 225-400 MHz military communications bands.
Further, some previous tunable patch antennas have incorporated varactor diodes into their substrate for the purpose of tuning. However, the tuning bandwidth is directly related to the ratio of the amount of electric energy stored in the tuning diode(s) to the amount of energy stored in the antenna""s substrate. As the substrate dielectric constant is increased in a patch antenna, the antenna""s physical size is reduced, but so is the tuning range. No varactor tuned microstrip patch antennas are known where a high substrate permittivity (∈r greater than 10) has been employed with both 1) an electrically small element (i.e. patch length L less than xcex/4 where xcex is the free space wavelength), and 2) a broadband tunable element with an octave or more of tuning range.
Another challenge for the antenna designer is to create a tunable antenna capable of handling medium to high power levels of 30 watts average power or more. For instance, the UHF fleet SATCOM radio systems can provide up to 135 Watts average power upon transmit in the 290 MHz to 320 MHz band. Varactor tuned patch antennas have historically been low power handling elements since the RF voltage applied across the diode causes harmonic distortion at sufficiently high voltages. This is because previous designs used one diode in a shunt circuit between the patch and ground. Accordingly, a design is needed that minimizes the RF voltage drop across any one diode.
Still further, a major limitation of high power-handling tunable antennas is the need for significant control power. State-of the-art tunable antennas which handle CW power of up to 30 watts use PIN diodes which must be forward biased with typical currents of 10 to 100 mA. The tunable patch antenna disclosed in U.S. Pat. No. 5,777,581, for example, consumes between 3 and 10 watts of DC control power depending on the tuning state. It would be very advantageous to develop an alternative tunable antenna technology which uses much less control power.
The present invention is related to a tunable artificial dielectric material that achieves the weight reductions made possible in U.S. Pat. No. 6,075,485 and further achieves even higher resonant frequency tuning ratios. In one embodiment of the invention, the artificial dielectric substrate for a patch antenna comprises alternating low and high permittivity layers, with the high permittivity layers each comprised of printed capacitive Frequency Selective Surface (FSS). The FSS of the invention has a voltage tunable effective sheet capacitance by virtue of varactor diodes integrated into each unit cell. By appropriate adjustment of the bias voltage across the varactor diodes, the amount of the electric field stored in the substrate can be varied, which further varies the resonant frequency of the patch antenna.
The present invention is particularly useful for UHF fleet SATCOM applications where a light weight and physically small (8xe2x80x3 sq. aperture) conformal aperture is desired as a mobile platform such as a military ground vehicle, fighter aircraft, or helicopter. Since the tuning bandwidth approaches one octave in this invention, 225 MHz to 400 MHz military UHF line-of-sight (LOS) communications applications are possible.